This invention relates to improvements in aqueous clay-based drilling fluids which better enable drilling at high temperatures by decreasing fluid loss. Aqueous clay-based drilling fluids or muds are well-known in the prior art, as are different additives. Such fluids are comprised basically of water, a clay such as bentonite or sepiolite, lignosulfonate, a base such as NaOH, a densifier such as barite, or barium sulfate and possibly a salt containing a cation such as sodium or potassium. Other aqueous ionic compounds such as Na.sup.+ Cl.sup.- may also be present. These fluids are suitable for drilling at pH levels from about 8 to about 11.5.
As drilling temperatures in recent years have been increasing, the prior art has searched for additives to conventional drilling fluids to facilitate drilling at higher temperatures. One major problem some of these additives have tried to alleviate has been an increased fluid loss due to high-temperature drilling. U.S. Pat. No. 4,172,800, issued to Walker, discloses an additive mixture of a polyethoxylated sulfurized fatty acid and a polyalkylene glycol. U.S. Pat. No. 4,201,678, issued to Pye et al, discloses a foaming solution suitable for drilling at up to 700.degree. F., containing an amphoteric betaine, a salt of a linear aliphatic or alkyl aryl hydrocarbon sulfonate, and, optionally, an unneutralized ammonia. U.S.S.R. Patent 883,140, issued to Boreholes Consolidated, discloses a drilling fluid containing a lignosulfonate, an acrylic polymer, potash alum, potassium hydroxide, and water. This fluid is used at temperatures up to 200.degree. C. and at pH levels from 8 to 12.9. U.S. Pat. No. 4,478,727, issued to Turner et al, discloses an additive consisting of a copolymer of sodium styrene sulfonate-co-sodium-N-(4-sulfophenyl)-maleimide.
Another additive which has been used in drilling fluids is a sulfonate amide copolymer. U.S. Pat. No. 4,309,523 to Engelhardt et al discloses copolymers consisting essentially of a random distribution of (a) 5 to 95% by weight of units of the formulae ##STR2## (b) 5 to 95% by weight of units of the formula ##STR3## and (c) 0 to 80% by weight of units of the formula wherein R.sub.1 and R.sub.2 are the same or different and each is hydrogen, methyl, or ethyl, and X.sup.+ is a cation. The terpolymer composition of the Engelhardt patent has been used in a drilling fluid as disclosed in U.S. Pat. No. 4,440,649 to Loftin et al. Loftin discloses the use of the composition in the Engelhardt patent as a fluid loss reducing and rheology stabilizing agent, either alone or in combination with pregelatinized starch and/or hydroxyethylcellulose. Loftin also uses organic cationic clay-stabilizing agents as well as water and a viscosity increasing agent. Loftin ages his fluids at temperatures up to 350.degree. F. The fluid loss experiments disclosed in the Loftin reference are standard API fluid loss experiments carried out at room temperature. This test, as described in Gray and Darley, Composition and Properties of Oil Well Drilling Fluids (1980), pg. 20, subjects the mud to static filtrations for 30 minutes, after which the volume of filtrate is measured through filter paper
R. E. Loftin, A. J. Son, and T. M. Ballard describe the use of a drilling fluid containing a vinylamide/vinylsulfonate polymer at temperatures in excess of 400.degree. F. in "Temperature-Stable Polymeric Fluid-Loss Reducer Tolerant to High Electrolyte Contamination," a paper presented at the 59th Annual Technical Conference and Exhibition at Houston, Tex., on Sept. 16-19, 1984. The paper was published by the Society of Petroleum Engineers of AIME and is referred to as SPE 13160. The fluid loss tests, however, as in the Loftin patent, are API fluid loss tests done at room temperature and not at the drilling temperatures of 400.degree. F. (about 200.degree. C.) and above.
Problems encountered in the prior art include increased fluid loss at high temperatures and/or the decomposition of some drilling fluid additives at high temperatures. The object of the present invention is to alleviate both of these problems with a new additive system for drilling fluids.